Operating System Structure

 

Multiprogramming needed for efficiency

Single user cannot keep CPU and I/O devices busy at all times

Multiprogramming organizes jobs (code and data) so CPU always has one to execute

A subset of total jobs in system is kept in memory

One job selected and run via job scheduling

When it has to wait (for I/O for example), OS switches to another job

Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing

Response time should be < 1 second

Each user has at least one program executing in memory [process

If several jobs ready to run at the same time [ CPU scheduling

If processes don’t fit in memory, swapping moves them in and out to run

Virtual memory allows execution of processes not completely in memory